A phosphor is a luminescent material that absorbs radiation energy in one portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. One important class of phosphors includes crystalline inorganic compounds of very high chemical purity and of controlled composition, to which small quantities of other elements, called “activators,” have been added for fluorescent emission. With the right combination of activators and inorganic compounds, the color of the emission of these crystalline phosphors can be controlled. Most useful and well known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic energy outside the visible range. Well known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (UV) radiation emitted by the excited mercury to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons, useful in photomultiplier tubes, or X-rays, such as scintillators used in imaging systems.
One important property of phosphors is the decay time, e.g., the time required for the phosphor to stop emitting light after the excitation is removed. Most phosphor compositions have extremely short decay times, with most of the stored energy emitted as light within seconds, or even a small fraction of a second after excitation ends. These phosphors may be useful in lighting type applications where continuous excitation is present. However, in many applications it would be worthwhile to have a phosphorescent material that continues to emit light for long periods of time after excitation has ended. Phosphors based on ZnS compositions were developed to fill this need, but these materials have a number of problems, including low stability, poor color, and a relatively short decay time.
In the past, short decay times were overcome by the use of radio luminescent paint. Radio luminescent paint consists of a radioactive power source mixed with a phosphorescent crystalline pigment. The earliest materials, starting in the early 1900s, were based on the use of radium as the power source. These paints also contained zinc sulfide (ZnS) as a scintillator material, which converted the high energy radioactivity into visible light. These systems had the advantage of being permanently powered luminescent systems, with no need for charging in sunlight or other lighting sources. However, the systems had a number of limitations which led to their use being dramatically reduced or stopped by the late 1970s. Specifically, the majority of the radioactive materials involved were highly toxic, and led to significant doses of radioactivity, both to the manufacturing personnel and to the users of the objects. Newer phosphorescent materials have been developed in an attempt to provide a low-toxicity material with a relatively long decay time. However, few of these materials have decay times of more than a few hours. Thus, upon charging, the materials will visibly glow for two or three hours after the light source is removed, and then fade to the point where they can no longer be seen.
Accordingly, there is a need for a new phosphorescent material that would have low toxicity in comparison to previous phosphorescent materials, and yet would have a decay time of several hours.